1. Field of the Invention
The invention relates to a plasma-activated CVD (chemical vapor deposition) process for coating a basic tool body, preferably made of hard metal or steel, with at least one surface layer composed of hard substances, e.g., at least one of carbides, nitrides and/or carbonitrides of titanium and/or zirconium. The invention also relates to a tool produced by a method including the plasma-activated CVD process, and to the process of cutting plastic sheets coated with magnetic materials into magnetic tape suitable for the storage of audio or video signals using the aforementioned tool configured as a circular shearing blade.
2. Description of the Related Art
The German periodical Angewandte Chemie [Applied Chemistry], Volume 69, No. 9, pages 281-312, (May 7, 1957), describes circumventing the brittleness of hard substances by applying the hard substances in the form of thin surface coatings in order to protect recipient substrates against mechanical or chemical stresses. The article mentions, for example, the coating of sheet metal with a 6 .mu.m coating of TiN.
Additionally, Swiss Patent No. 507,094 discloses, for example, a molded body composed of a hard metal substrate and at least one hard substance layer, with the hard metal substrate being composed of one or a plurality of hard substances and at least one binder metal, and the hard substance layer(s) including hard carbides or nitrides. Since such molded bodies have good wear characteristics, they can be employed for tools used in cutting, as well as noncutting shaping. The molded bodies are produced according to the CVD process as it is disclosed, for example, in Swiss Patent No. 452,205. At present, the CVD process is one of the most commonly employed coating processes and involves depositing a surface layer on a substrate from a reactive gas atmosphere which generally has a temperature between 900.degree. and 1,200.degree. C. The gas atmosphere includes several compounds which react with one another at the reaction temperature and form the substance comprising the surface coating by precipitation thereof onto the substrate.
Today, it is customary to coat metal substrates with hard substance layers composed of carbides, nitrides or carbonitrides, with the gas atmosphere including halogenides of elements of Groups III to VI of the Periodic Table of Elements, a nitrogen-containing compound, and a carbon containing compound. For example, a titanium carbide layer may be deposited at about 1,000.degree. C. on a basic hard metal body from a gas atmosphere which includes titanium tetrachloride and methane. Gaseous hydrocarbons, in particular, are employed as the carbon-containing compounds, while typical nitrogen-containing compounds employed include N.sub.2, NH.sub.3 or amines. However, the high coating temperatures of about 1,000.degree. C. cause the composite body to lose toughness.
The German periodical VDI-Z, Vol. 124, No. 18, (September, 1982 (II)), pages 693 et seq., describes the application of wear resistant layers on tool steel by means of a CVD process. Again, the tool to be coated must remain at a high temperature for a relatively long time and the dangers arise of development of crystals in the steel and disadvantageous stabilization of the austenitic phase, the high temperature phase of steel, rather than reversion to the martensitic phase on cooling. At the conclusion of the CVD process, the reactor is cooled under hydrogen at a cooling rate ranging from 20.degree. to 50.degree. C./min. Steels which are hardened by being quenched in water or oil, however, do not reach their maximum hardness under these conditions. This again confirms that high CVD temperatures bring about undesirable side effects.
In order to provide a lower deposition temperature, a process referred to as the plasma-supported CVD process has been proposed. In a plasma of a low pressure glow discharge, the gas mixture is composed of neutral particles, molecules, dissociated molecules, ions and electrons. Due to the lack of an equilibrium state during the low pressure discharge, the temperature of the electrons is several thousand degrees higher than that of the heavy particles, i.e., ions and neutral particles. This causes activation of the chemical reaction(s) of the CVD process and at temperatures below the 1,000.degree. C. otherwise required in a CVD process. A low pressure plasma may be generated using DC or HF sources, for example, by application of a DC voltage (hereinafter referred to as a direct voltage) or a high frequency AC voltage, at pressures ranging from 10 to 1,000 Pascal.
The simplest way to produce a low pressure discharge is to connect the workpiece to be coated to the cathode of a direct current source and to connect the walls of the CVD chamber, at least a portion of which are electrically conductive, to the anode of the direct current source. However, the more expensive high frequency process can be operated without electrodes and, advantageously, metal, as well as non-metal, substrates can be coated with this process. The substrate temperature is, moreover, a function of the high frequency energy introduced. However, this process is very complicated and relatively expensive.
European published Application No. 0,199,527 A1 additionally discloses a plasma CVD process in which a negative direct voltage of up to 1,500 volts is applied to the substrate to be coated to excite the plasma and superposed on this direct voltage is a high frequency voltage at, for example, 13.56 MHz.
Federal Republic of Germany published Application No. 3,027,688 B1 discloses a process in which, subsequent to application of a surface coating, a negative direct voltage is applied to the composite body in an inert gas atmosphere at a pressure ranging from 10.sup.-2 to 10.sup.2 mbar and at a temperature ranging from 200.degree. to 800.degree. C. These conditions suffice to maintain a glow discharge for at least ten minutes.
Federal Republic of Germany published Application No. 601,847 A discloses a process for heat treating metals, particularly for carboreting iron or tungsten, respectively, in a partial vacuum in which the glow discharge is a sudden discharge of a capacitive energy store, i.e., is generated by short-term voltage pulses which have a pulse duration of about 10.sup.-5 to 10.sup.-6 seconds. The pause intervals between the individual pulses are selected to be at least ten times as long as the pulse duration itself so that the gas path can be deionized in the meantime.